# 4133
Since almost the start of the novel H1N1 outbreak last spring, one of the major topics of discussion has been the severity – as measured by the CFR (Case Fatality Ratio) – of this virus.
In other words, what percentage of those infected (and symptomatic) are likely to die?
It is not an easy question to answer, and even though it appears that this virus will go down as `relatively mild’, we are only six months into this outbreak, and the virus could mutate or change over time.
Thus far, however, we seem to be dealing with a high morbidity - low mortality pandemic. One that – due to a lack of widespread immunity – causes a lot of illness. But one that doesn’t cause a lot of deaths.
Good news, by almost any measure.
But there are caveats.
Those that are dying from this virus – even if their absolute numbers are less than a normal flu season – tend to be younger adults and children. Something quite unusual when it comes to seasonal flu, which typically claims the elderly and frail.
While exact numbers are impossible to compile, researchers have published new estimates of the CFR of the novel H1N1 in yesterday’s PLoS Medicine journal. These numbers give us a range of likely values, not a single number writ in stone.
First, portions of the press release from the Harvard School of Public Health, then some excerpts (slightly reparagraphed) from the PLoS Medicine study.
I’ll have a few closing remarks as well.
Severity of H1N1 flu in US during current flu season may be less than feared
Burden of disease may fall on young children and non-elderly adults
Boston, MA -- A new study from researchers at the UK Medical Research Council and the Harvard School of Public Health (HSPH) projects that the severity of the H1N1 flu during the autumn-winter flu season in the U.S. will likely be less than previously feared. The estimates of hospitalizations and life-threatening events in the study are the most accurate to date of the H1N1 pandemic's impact in the U.S. The study appears online on December 7, 2009 in the journal PLoS Medicine.
<SNIP>
The researchers used two different approaches to estimate these risks. One approach led to an estimate that approximately 1.44% of patients with symptoms of H1N1 flu during the April-July time period were hospitalized, 0.239% required intensive care or mechanical ventilation and 0.048% died. The other estimate found probabilities 7 to 9 times lower, due to the use of different data to estimate how many individuals were sick. Based on these findings and assuming that the virus doesn't change its characteristics, the researchers estimate that the severity of the autumn-winter pandemic wave of H1N1 flu could have a death toll in a range from considerably below the estimated 36,000 associated with an average flu season in the U.S. to slightly higher. Also, unlike seasonal flu, which kills mainly elderly adults, the H1N1 flu could have the greatest impact in children aged 0-4 and especially adults 18-64, a shift toward nonelderly persons that has been seen in prior flu pandemics.
"The good news is that, along with previous work by the CDC and others, our work shows that the severity of the H1N1 flu may be less than initially feared," said Lipsitch. However, he adds that between 1 in 70 and 1 in 600 people who are sick with the illness will be hospitalized, and a fraction of those will die.
A preliminary version of this article was posted on PLoS Currents (September 25th 2009), a new forum for rapid publication of results designed to aid timely dissemination of important information during the pandemic.
Support for the study was provided by the UK Medical Research Council, the UK Health Protection Agency, the U.S. National Institutes of Health and Department of Homeland Security.
The Severity of Pandemic H1N1 Influenza in the United States, from April to July 2009: A Bayesian Analysis
Marc Lipsitch and colleagues use complementary data from two US cities, Milwaukee and New York City, to assess the severity of pandemic (H1N1) 2009 influenza in the United States.
Anne M. Presanis, Daniela De Angelis, The New York City Swine Flu Investigation Team,, Angela Hagy, Carrie Reed, Steven Riley, Ben S. Cooper, Lyn Finelli, Paul Biedrzycki, Marc Lipsitch
<EXCERPT>
What Did the Researchers Do and Find?
By using data on medically attended and hospitalized cases of pH1N1 infection in Milwaukee and information from New York City on hospitalizations, intensive care use, and deaths, the researchers estimate that the proportion of US cases with symptoms that died (the sCFR) during summer 2009 was 0.048%.
That is, about 1 in 2,000 people who had symptoms of pH1N1 infection died. The “credible interval” for this sCFR, the range of values between which the “true” sCFR is likely to lie, they report, is 0.026%–0.096% (between 1 in 4,000 and 1 in 1,000 deaths for every symptomatic case). About 1 in 400 symptomatic cases required treatment in intensive care, they estimate, and about 1 in 70 symptomatic cases required hospital admission.
When the researchers used a different approach to estimate the total number of symptomatic cases—based on New Yorkers' self-reported incidence of influenza-like-illness from a telephone survey—their estimates of pH1N1 infection severity were 7- to 9-fold lower. Finally, they report that the sCFR and the sCIR were highest in people aged 18 or older and lowest in children aged 5–17 years.
Critics will no doubt point out that the CFR estimated from this study ranges from 1 in 1,000 to 1 in 14,000 – depending on which method is used.
The use of self-reported ILI (via phone surveys) casts a wide net , and is likely to catch a lot of non-influenza flu-like illnesses. That would skew the numbers badly, making the severity of the virus appear artificially lower.
Even during the height of flu season, the percentage of samples submitted to the CDC for analysis that come back positive for flu rarely exceeds 30%. This past week, only 15% of the samples tested came back as positive for Influenza.
Basing your numbers on medically attended and hospitalized cases excludes mildly symptomatic cases, and those who did not seek medical help. That would tend to make the virus appear more severe.
Neither method is ideal.
But for now, this may be as close as scientists can come to calculating the CFR of this virus.
For younger adults and children, the fatality rate is several times higher than for seasonal flu. For those over 65 – who traditionally suffer the most from influenza – the impact is far less than normal.
As far as what the `right’ CFR number is. We may never know exactly.
But over time, scientists will attempt to refine it further.
